In recent years, many packet scheduling algorithms have been proposed to reduce congestion, minimize delay (i.e., latency), and maintain fairness, especially to accommodate a high number of packet flows. Unfortunately, many of these algorithms can only be applied to fixed-size packets. Furthermore, many of these algorithms (even if they do not require fixed sized packets) exhibit poor performance as the number of packet flows increases.
Deficit Round Robin (DRR) is an algorithm which allows for variable-sized packets. Under DRR, a node rotationally selects packets to send out from all flows that have queued packets. During a round, each flow accumulates credits in discrete increments (e.g., in bytes) called a quantum. Unfortunately, DRR typically requires a quantum value that is very large, i.e., many times the size of the maximum packet size for a flow. The data presented in Table 1 below illustrates the above problem.
TABLE 1THE TRAFFIC PARAMETERS AND QUANTUM SIZE OF 4 FLOWSReservedMaximumRatio of MaxFlowRateTrafficPacket SizePacket Size toQuantumID(Mbps)Type(byte)Reserved RateSize (byte)A128CBR400250512B16CBR640320640C64CBR8001002560D64CBR1001252560
The data in Table 1 assumes four flows, sharing the same link, and a link capacity of 160 megabits per second. As illustrated in Table 1, the quantum size of a flow can reach very large values relative to the flow's maximum packet size. For example, flow D has a quantum size of 25.6 times (2560/100) the maximum packet size. Unfortunately, due to the large quantum size required by DRR, DRR's performance can be poor compared to other algorithms such as Self-Clocked Fair Queuing (SCFQ) and Weighted Fair Queuing (WFQ).
However, simply reducing the quantum size also creates problems and is generally ineffective. For example, reducing the quantum size can cause a node using DRR to select no packets to send out after querying all flows in a particular round. This causes unacceptable delay and, again, causes poor performance. Thus, simply reducing the quantum sized used in a DRR node is generally not effective. Accordingly, it would be desirable to provide a scheduling algorithm and apparatus which does not require fixed size packets and exhibits good performance, including when there is a high number of packet flows.